1. Field of the Invention
The present invention relates to a waveguide, a slotted antenna, and a horn antenna preferably for use in a millimeter wave-based onboard radar apparatus, in particular, a digital beam forming (DBF) radar that monitors the direction of travel of an automobile.
2. Description of the Related Art
A DBF radar includes a receiving antenna array composed of a plurality of receiving antenna elements arranged at predetermined intervals (typically, regular intervals) in a scan direction. The DBF radar converts received signals from each receiving antenna element into digital data, performs arithmetic processing on the digital data to impart a phase shift to each received signal, and synthesizes the phase-shifted received signal to generate an equivalent scan beam. The DBF radar can scan at high speed with high precision without the need for any drive part or movable mechanism and therefore is widely used as the onboard millimeter wave radar. However, the DBF radar requires a measure to prevent erroneous detection due to the grating lobe phenomenon.
Japanese Patent Laid-Open No. 2012-147105 discloses a patch antenna unit including transmitting antennas successively displaced and longitudinally symmetrically arranged in a V shape. Side lobes are reduced by using null characteristics of the V-shaped arrangement. However, if the patch antenna is supplied with electric power via a micro-strip line, the dielectric loss is high in the frequency band of the millimeter wave. If a waveguide were used for electric power supply, the loss would be reduced. However, there has not been known any method of supplying electric power to the V-shaped antenna array through a waveguide.